In 1973, Gross, working with his first graduate student, Frank Wilczek, at Princeton University, discovered asymptotic freedom, which holds that the closer quarks are to each other, the less the strong interaction (or color charge) between them; when quarks are in extreme proximity, the nuclear force between them is so weak that they behave almost as free particles. Asymptotic freedom, independently discovered by David Politzer, was important for the development of quantum chromodynamics.
Gross, with Jeff Harvey, Emil Martinec, and Ryan Rohm also discovered heterotic string.
Scientific articles (http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+EA+GROSS%2C+DAVID+J+OR+EA+GROSS%2C+D+J) of David Gross (SLAC database)